We propose to develop and demonstrate a radiationreflecting thermal protection system (TPS) to enable missions where radiative heating during atmospheric entry is significant, such as into a giant gas planet. Because layered or ordered structures can be difficult to fabricate, we will utilize a naturally occurring ordered material, diatoms. Diatoms are unicellular algae that grow siliceous structures. Tunable reflectors (for different planetary environments) can be constructed based on photonic structures, which are periodic arrangements of materials with contrasting dielectric properties. In such structures, internal reflections interfere destructively, blocking propagation of radiation into the material. Typically, this periodicity must be on the order of the wavelength to be reflected, which for EDL applications could be hundreds of nanometers. Fabrication of such small structures is very expensive, when possible at all. Our approach involves incorporating ordered entities into a matrix to form a material that will reflect radiation. We will use naturally occurring inexpensive diatoms to produce materials with ordered structures. The amorphous silica cell walls of diatoms are lightweight structures with a complex architecture that contains a periodic arrangement of pores in the micrometer to nm- range. Architecture of this complexity has not yet been achieved in materials science. Diatom pore arrangements are of the order of optical wavelengths; thus the cell walls act as photonic structures. The SiO2 cells will be converted to high temperature materials without destroying their inherent pore structure. The converted diatoms (CD) will be randomly incorporated in a matrix such as phenolic or a phenolic-impregnated composite material. The random orientation will permit reflection from a wide range of angles. Reflecting even part of the radiation can make the difference in making a TPS capable for a demanding mission. It may also be possible to orient the entities to provide preferred directions of reflection.